The invention relates to a method and an arrangement for monitoring an exhaust system.
Exhaust systems are used for breaking down the pollutant fractions of the exhaust gases that are generated during the operation of an internal combustion engine. The remaining exhaust gases are then discharged. The exhaust system is generally composed of multiple parts, specifically one or more silencers, pipes and a catalytic converter.
Catalytic converters in particular serve for exhaust-gas treatment or for aftertreatment in order to reduce pollutant emissions in the exhaust gas. Here, various types of catalytic converters are known, such as for example three-way catalytic converters, unregulated catalytic converters, oxidation catalytic converters and SCR catalytic converters. In SCR catalytic converters, for example, use is made of so-called selective catalytic reduction (SCR) as a method for the reduction of nitrogen oxides.
To monitor the functionality of the catalytic converter, various methods are known.
Document DE 40 27 207 A1 describes a method for monitoring the catalytic activity of a catalytic converter in the exhaust system of an internal combustion engine, in which method signals that are output by measurement sensors assigned to the catalytic converter are evaluated. Here, signals from at least two measurement sensors are recorded, the detected measurement variables are observed over a relatively long time period, and an average value is formed. The catalytic activity is determined by comparison of the mean value with a predefined threshold value.
Document DE 43 08 894 A1 discloses a method for testing the conversion of a catalytic converter in motor vehicles with internal combustion engines. The testing is performed by means of a temperature measurement upstream and downstream of the catalytic converter, wherein a temperature difference is determined. The testing is in this case performed during overrun operation of the motor vehicle. The catalytic converter is regarded as functional if, inter alia, the temperature difference increases and the temperature downstream of the catalytic rotor lies in a predefined range during the test.
Document DE 42 11 092 A1 describes a method and a device for assessing the functionality of a catalytic converter. In said method, use is made of the knowledge that the temperature at which conversion begins in the catalytic converter rises with the degree of aging of the catalytic converter.
Document US 2011/0143449 A1 describes a method for monitoring an exhaust system, in which method it is established whether or not a catalytic converter is present. For this purpose, a measured time delay is compared with an estimated time delay.
To check whether a catalytic converter has been removed, it is known to arrange a differential pressure sensor across the catalytic converter. Said differential pressure sensor has the task of identifying whether a catalytic converter is installed, or whether said catalytic converter has for example been removed by the operator. In many applications, the differential pressure sensor is installed only for the purposes of identifying an empty pipe. This incurs additional costs.